ISSN 0869 5938, Stratigraphy and Geological Correlation, 2011, Vol. 19, No. 5, pp. 502–514. © Pleiades Publishing, Ltd., 2011. Original Russian Text © V.V. Mitta, V.A. Zakharov, I.S. Barskov, V.B. Sel’tser, A.V. Ivanov, 2011, published in Stratigrafiya Geologicheskaya Korrelyatsiya, 2011, Vol. 19, No. 5, pp. 32–45.
The Upper Bajocian–Lower Bathonian Boundary Section in the Outskirts of Saratov: Molluscan Characteristics and Biostratigraphy V. V. Mittaa, V. A. Zakharovb, I. S. Barskova, V. B. Sel’tserc, and A. V. Ivanovc a Borissiak Paleontological Institute, Russian Academy of Sciences, Profsoyuznaya ul. 123, Moscow, 117647 Russia e mail: [email protected] b Geological Institute (GIN), Russian Academy of Sciences, Pyzhevskii per. 7, Moscow, 119017 Russia c Chernyshevskii Saratov State University, Astrakhanskaya ul. 83, Saratov. 410012 Saratov Received February 4, 2011; in final form April 13, 2011
Abstract—The analysis of all available data on the structure of the Bajocian–Bathonian boundary section in the outskirts of Saratov (Sokur quarry) and the taxonomic composition of its ammonites, belemnites, and bivalves revealed a continuous succession of the Pseudocosmoceras michalskii (Upper Bajocian), Oraniceras besnosovi, and Arcticoceras ishmae (Lower Bathonian) zones. In connection with the critique by Meledina et al. (2009), correlation of Bajocian and Bathonian boundary strata of the Central Russia and Northern Siberia is discussed. The inconsistency of Siberian bivalve and belemnite assemblages with Central Russian ammonite zones is explained by heterochronous invasions of different molluscan groups.
Keywords: Saratov Volga region, ammonites, Bajocian, Bathonian, biostratigraphy, zones, Boreal–Tethyan correlation. DOI: 10.1134/S086959381105008X
INTRODUCTION sediments in the Saratov–Volgograd region (Lower Volga River). A.V. Pavlov, a follower of Professor The Bajocian–Bathonian sequences of the Sara A.P.Pavlov, was the first to discover differences tov–Volgograd Volga region containing rare ammo between the Jurassic deposits of the Saratov region and nites of the family Parkinsoniidae have been studied typical Central Russia Jurassic sections, revealing for a century. Discovery of these Tethyan ammonites in members of light gray clays with intercalations of the quarry near Saratov (Mitta and Sel’tser, 2002) clayey siderite in the Saratov province, the so called together with Boreal Arctocephalitinae (family Cardi “Upper Dogger with Parkinsonia parkinsoni Sow.” oceratidae), which had previously been considered to (Pavlov, 1904a, 1904b). Rzhonsnitskii (1905) estab be exclusively a high latitude group, renewed interest lished the Dogger strata with Parkinsonia representa in this region. Recent years have produced new data tives below the Lower Callovian sediments with on the distribution of fossil remains in this section Cadoceras elatmae in the Chardym River basin (Sara (Mitta et al., 2004; and others). In addition, a bore tov Volga region). hole, which recovered older Bajocian strata, has been drilled in this area. Recently, Meledina et al. (2009) Arkhangelsky (1906) noted that in the Kamyshin published a paper with a critique of our views on the district of the Saratov Region, dark clays with clayey Boreal–Tethyan correlation of the Bajocian–Batho siderite concretions containing abundant Lower Call nian boundary beds. In this work, we discuss both ear ovian ammonites such as “Kepplerites aff. Gowerianum lier and recent data, and respond to our opponents. Sow., Cardioceras Chamussetti d’Orb., Cadoceras Voucher material discussed in this work is housed at surense Nik., C. Elatmae Nik., and C. Frearsi d’Orb.” the Borissiak Paleontological Institute (PIN) of the overlie pale sandy–micaceous platy clays with interca Russian Academy of Sciences and Vernadskii State lations of thin bedded micaceous sandstones contain Geological Museum (SGM) of the Russian Academy ing rare poorly preserved Pseudomonotis remains of Sciences. attributed to undivided Bathonian–Callovian interval. They are underlain by Bathonian layers separated in two members: HISTORICAL REVIEW (1) dark clays with gypsum and marcasite interca Until the end of the 19th century, the Callovian lated by beds and concretions of dark clayey siderites strata were considered to be the oldest Jurassic marine with Parkinsonia and abundant Pseudomonotis; the
502 THE UPPER BAJOCIAN–LOWER BATHONIAN BOUNDARY SECTION 503 lower clays enclose beds of ferruginous sandstone and described slightly later by P.K. Murashkin, who pro pebble gravel consisting of rounded fragments of Car vided the first image of ”Parkinsonia (?) mojarowskii boniferous limestones and cherts; Masar.” (holotype by monotypy). In addition, this (2) white calcareous sandstones, yellow quartz and author described Parkinsonia subcompressa sp. nov. gray micaceous–glauconite sands with beds of pebble (based on a single poorly preserved fragment) and gravel and separate large cherty blocks; the base of defined the genus Pseudocosmoceras with the type spe sands immediately above the Carboniferous lime cies Cosmoceras michalskii Bor., that was first stones is represented by red ferruginous conglomerate. described by Borisyak (1908) from the Donets Jurassic Rzhonsnitskii’s studies of 1904–1906 and 1914 (Lower Bathonian after Borisyak, 1917). This genus made it possible to specify the structure of Jurassic also included “varieties” of the type species (Ps. sections in the outskirts of Saratov. Below, we cite his michalskii var. minor and var. media and Ps. masarowici inferences concerning the lower part of the section: nov. sp. (with var. descendens, var. conjungens, and var. “The oldest rocks constitute the Bathonian Stage, inclara). Murashkin finishes his article with the words represented by gray and yellowish gray clays with inter “…from my point of view it is not possible at present to calations of dark gray siderite. These rocks yielded estimate the exact age of beds with the above men ammonites belonging to the genus Parkinsonia, tioned ammonites. It is conceivable that beds with bivalves of the genera Pseudomonotis, Pleuromya, and Parkinsonia subcompressa nov. sp., correspond to the others, belemnites, and wood fragments. The most zone with Parkinsonia würtembergica Opp. (? = Par abundant fossils are single Pseudomonotis species kinsonia compressa Qu.) correlated with the Upper resembling P. echinata Sow., which literally oversatu Dogger in the Jurassic section of northwestern Ger rate both clays and siderite beds. … The Bathonian many. One may confidently postulate synchronism of strata are immediately overlain by gray and yellowish Kamyshin Middle Jurassic sediments containing the gray sandy and micaceous clays with intercalations of above mentioned varieties of Pseudocosmoceras thin bedded dark gray fine grained micaceous sand michalskii from the Donets Middle Jurassic beds con stones, which are readily separated into fine plates. taining Pseudocosmoceras michalskii Bor. var. typica” These sediments are mostly barren of organic remains, (Murashkin, 1930, p. 159). In his table of “assumed while their upper part contains rare Lower Callovian phylogenetic relationships between examined ammo Cadoceras forms, which allow the entire sequence to nites” this author indicates the “Parkinsonia donezi be attributed with a certain degree of confidence to the ana Zone with hypothetical forms closely related to Lower Callovian. … These strata are overlain by black this species” and higher, beds with Pseudocosmoceras gypsum bearing clays with spheroid concretions and michalskii and Ps. masarowici, varieties included intercalations of dark gray siderite and intergrowth of (Murashkin, 1930, p. 150). ferrous pyrite. These clays contain abundant In 1930, Murashkin conducted fieldwork in the Cadoceras modiolare Sow., Cadoceras Elatmae Nik., Kurdyum and Chardym river basins (North of Sara Cadoceras Frearsi Nik., Macrocephalites sp., Cardio tov), where he noted “gray, locally, ferruginous sandy d’Orb., ceras Chamussetti Kepplerites Gowerianum clays with lenticular beds of siderites containing Par Sow., belemnites, Gryphaea, wood fragments, and kinsonia compressa var. close to var. würtembergica others” (Rzhonsnitskii, 1914, p. 60). Opp. emend. Nicolesco” attributed to the Bathonian In 1917, Jurassic sections in the Ilovlya River basin Stage (Murashkin, 1932, p. 74). were visited by A.N. Mazarovich, who noted under “Neocomian sands” in the Kamyshin district, thin In the middle of the 20th century, a group of Sara Oxfordian, “Callovian, problematic Bathonian, Bajo tov researchers headed by V.G. Kamysheva cian,” and underlying strata, which are divisible into Elpat’evskaya systemically investigated Jurassic fossils two units: the Karaulinskaya Group and Gnilushki in the Saratov Volga region. In several guidebooks and conglomerates and sands overlying the Carboniferous atlases (Kamysheva Elpat’evskaya and Ivanova, 1947; limestones” (Mazarovich, 1923, p. 31). Mazarovich Kamysheva Elpat’evskaya et al., 1956, 1959), these reached the conclusion that clays with siderite concre researchers described and illustrated (in most cases as tions underlying “clayey sands, siderites, and pale Bajocian) some Bathonian Parkinsoniidae. They had clays of the problematic Bathonian” are older, and limited material at their disposal (approximately attributed them to the Bajocian. He assumed that the 10 specimen), which prevented a comprehensive Bajocian Stage is represented by two zones: lower investigation of the taxonomic composition of the fos Stephanoceras humphriesianum and upper Parkinsonia sils. Clays with intercalations of siderite concretions parkinsoni. From the lower zone, he described a new were ambiguously dated: in the explanations of the loosely collected ammonite species Sonninia plates Parkonsonia forms in the above mentioned mojarowskii. works were attributed to the Bajocian, while in their These and other ammonites from the collection by subsequent summarizing work, the authors referred Mazarovich (from clays with intercalations of siderite the Parkinsonia to the Bajocian and Pseudocosmoceras concretions universally developed in the northern and Medvediceras, to the Lower Bathonian (Atlas…, extremity of the Don–Medveditsa swell) were 1969).
STRATIGRAPHY AND GEOLOGICAL CORRELATION Vol. 19 No. 5 2011 504 MITTA et al.
° 40 E Nizhnii Novgorod Bartolomeevskii . R. R a Volg ka Moscow O Atamanovka
RUSSIA Dubki
Saratov 199 D on 50°N Zharenyi bugor R. UKRAINE Volgograd Raskovo
Vo lga R. Rostov on Don Saratov Sea of 0300 km 04 km Azov
Fig. 1. Schematic map of the Lower Volga region. Star designates the position of the Sokur section in the northwestern Saratov outskirts.
Some images of Parkinsoniidae specimens from the (Pechora River basin, North Siberia, Arctic Canada, area under consideration have been published in other and East Greenland), were first found in a single sec works (Kamysheva Elpat’evskaya, 1951; Sazonov, tion together with Peri Tethyan Parkinsoniidae. Sub 1957; Sazonova and Sazonov, 1967). Using finds from sequently, the results of complex biostratigraphic stud the Ilovlya and Medveditsa river interfluve, Nikolaeva ies and data on the taxonomic composition of ammo 1 nites, belemnites, bivalves, gastropods, and (1967) described the new genus Medvediceras with palynological complexes were published (Mitta et al., the type species Ps. masarowici Murashkin. 2004). Several recent works contain descriptions of It should be noted that most of the ammonites some ammonites from the Volga region near Saratov illustrated in the publications cited were either loosely with substantiation of Boreal–Tethyan correlation collected or found in isolated outcrops, which pre between Bajocian–Bathonian sections also using vented their succession in the section from being material from the Pechora River basin (Izhma River) established. Numerous ammonites, which were deter (Mitta, 2004, 2007, 2009). mined in boreholes, were never illustrated and are missing from collections. THE SOKUR SECTION The recent phase in the study of Bajocian–Batho nian sections in the area under consideration is con The Sokur section (Fig. 2) contains the following nected with the section recovered in the clay quarry in beds (from the quarry bottom upward) (Mitta and the northwestern outskirts of Saratov 0.4 km away Sel’tser, 2002, 2009; Mitta et al., 2004): from the Sokur Road; therefore, it is known among geologists as the Sokur or Sokur Road quarry (Fig. 1). The clays with siderite concretions yielded abundant Upper Bajocian Parkinsoniidae and single Macrocephalitidae. The Pseudocosmoceras michalskii Zone subsequent detailed study of available collections and (1) Clay, light gray with the bluish tint, with insig new finds revealed that Arctocephalites and Arctico nificant silt admixture, separating into platy frag ceras belonging to the subfamily Arctocephalitinae of ments. The apparent thickness is approximately 5 m. the family Cardioceratidae were erroneously identi Sediments 2 m below the bed top and at the same level fied as Macrocephalitidae (Mitta and Sel’tser, 2002). or slightly lower yielded Pseudocosmoceras masarowici These taxa, which are characteristic of high latitudes Murashkin (Mitta and Sel’tser, 2002, Plate 6, fig. 2) 1 According to Mitta (2009), Medvediceras Nikolaeva, 1967 is a and large (250 mm across) shell of Parkinsonia sp. (sp. junior subjective synonym of Pseudocosmoceras Murashkin, nov. ?), respectively, while the upper part of the bed 1930. provided small shelled Parkinsonia sp. juv. (Mitta
STRATIGRAPHY AND GEOLOGICAL CORRELATION Vol. 19 No. 5 2011 THE UPPER BAJOCIAN–LOWER BATHONIAN BOUNDARY SECTION 505 et al., 2004, Plate 1, fig. 5; this work, Plate II, fig. 3). harlandi faunal horizon of the Arcticoceras ishmae In addition, Belemnoidea indet. (juvenile rostrum Zone based on the appearance of Arcticoceras har with a part of phragmocone), shells of Tancredia sp., landi. Katosira? sp., and Caenogastropoda gen. indet. were found in situ. The bed is attributed to the Pseudocos moceras masarowichi faunal horizon. Arcticoceras ishmae Zone (4) Clay, gray, silty, with rare sideritic carbonate concretions. In the upper part of the bed, a thin lentic Lower Bathonian ular layer of clayey material contains abundant Oraniceras besnosovi Zone rounded belemnite rostra and fragments of pyritized wood with attached bivalve shells (the so called (2) Clay, light gray, with intercalations of sideritic “belemnite level”). The thickness is 0.35 m. The bed carbonate concretions at the base and top. The thick yielded diverse fossils: Nannobellus bellus Barskov, ness is 0.8 m. At the base of the bed, concretions con N. parabellus Barskov, Paramegateuthis cf. pressa Nal tain Oraniceras cf. mojarowskii (Masarowich); concre njaeva, P. cf. manifesta Nalnjaeva, Pachyteuthis subre tions from the upper part of the bed yielded O. cf. bes diviva (Lemoine), Modiolus (Modiolus) cf. bipartitus nosovi Mitta et Seltzer, Modiolus (Modiolus) cf. (Sowerby), Liostrea (Deltostrea) eduliformis (Schlo bipartitus (Sowerby). The bed corresponds to the theim), Liostrea (Liostrea) multiformis (Koch), Quen Oraniceras mojarowskii faunal horizon. stedtia sp., Protocardia sp., Goniomya cf. v scripta (Sowerby), Pleuromya ? sp., Berenicea sp. The basal Oraniceras besnosovi and Arcticoceras ishmae Zones part of the bed yielded Arcticoceras harlandi (Rawson) (Mitta and Sel’tser, 2002, Plate 1, fig. 1); Arctocepha (3) Clay, light gray to gray, with silt admixture lites freboldi Spath (Arctocephalites sp. in Mitta and increasing upward the section and sideritic carbonate Sel’tser, 2002, Plate 4, fig. 2) probably originate from concretions at the base and scattered through the sec the same level. The “belemnite level” is characterized tion. The thickness is up to 3 m. The basal beds enclose by Acticoceras ishmae (v. Keyserling) (Plate II, fig. 2) Oraniceras besnosovi Mitta et Seltzer, Oraniceras sp. and single ichthyosaurus vertebra fragment (Arkhan juv. (Plate II, fig. 5), Mesosacella ex gr. morrisi gel’skii and Pervushov, 2002). The bed is attributed to (Deshayes), Thracia (Thracia) depressa (J. de. C. So the Arcticoceras harlandi faunal horizon. werby), Th. (Thracia) sp., Modiolus (Modiolus) cf. (5) Clay, reddish gray, silty, grading to brownish bipartitus (Sowerby), Meleagrinella echinata (Smith), gray and yellow–brown thin bedded clayey silt locally Oxytoma (Oxytoma) inaequivalve (Sowerby), and Pro cemented into thin platy siltstone. A lenticular layer tocardia sp. Higher in the section, the following fossil 0.1–0.2 m above the base with rare sideritic concre
assemblages were found in sediments: (1) Parkinsonia tions and scattered rounded belemnite rostra yielded sp., Retroceramus sp. juv., Parvulacteon^ sokurensis Arcticoceras ishmae (v. Keyserling) (Mitta and Sel’tser, Gründel, Tricarilda cf. extenta (Jamnic enko), and 2002, Plate 3, fig. 1) and impression of ammonite Levipleura sp. 0.5–1.0 m above the base of the bed; identified as Parkinsonia (s.l.) (Mitta and Sel’tser, (2) Parkinsonia sp., (Plate II, fig. 2), Pachyteuthis aff. 2002, Plate 5, fig. 3). The thickness is 1.1 m. The bed
subrediviva (Lemoine), Camptonectes cf. rigidus (Sow is attributed to the Arcticocears ishmae faunal horizon. erby), Liostrea sp., Protocardia sp., Parvulacteon^ soku Gründel, cf. (Jamnic enko), rensis Tricarilda extenta ?Middle and Upper Bathonian Levipleura sp. and Serpula sp. 1.5–2.0 m above the base; (3) Oraniceras besnosovi (Mitta et Seltzer, Modi (6) Silt, yellowish gray, thin bedded, cemented into olus (Modiolus) cf. bipartitus (Sowerby), Liostrea mul either compact siltstone protruding from the quarry tiformis (Koch), Thracia (Thracia) depressa wall in form of a plate (at the base) or locally poorly (J. de. C. Sowerby), Pinna sp., Protocardia sp., Arc consolidated siltstone (in the upper part). The thick tica? sp., and Pleuromya cf. uniformis (Sowerby) 2.0– ness is approximately 6.5 m). No macrofossils are 2.5 m above the base; (4) Oraniceras sp., Arcticoceras found in situ, although some plates of yellowish brown harlandi Rawson (Plate I, fig. 5), Aequipecten sp., siltstone in talus are saturated with separated valves of Myopholas sp., and Camptonectes (Camptonectes) ex gr. Oxytoma (Oxytoma) ex gr. inaequivalvis (Sowerby). lamellosus (Sowerby) 2.5–3.0 m above the base. The specimens of Oraniceras besnosovi Mitta et Seltzer ?Lower Callovian illustrated in (Mitta and Sel’tser, 2002, Plate 5, fig. 1, Plate 6, fig. 3, Plate 7, figs. 2, 3; Mitta et al., 2004, (7) Clayey brownish gray, becoming yellowish gray Plate 1, fig. 1) and a single specimen of Sokurella upward the section, silty, with small gypsum crystals galaczi Mitta found in situ (collection by S.A. Bratash and jarosite inclusions. The apparent thickness ova) also originate from this bed. The most lower part amounts to 3.2 m. of the bed is attributed to the Oraniceras besnosovi fau As follows from the description, beds 1–5 are suffi nal horizon, while the upper 0.5 m are referred to the ciently well characterized by fossil organic remains
STRATIGRAPHY AND GEOLOGICAL CORRELATION Vol. 19 No. 5 2011 506 MITTA et al. suitable for dating host sediments. It should be (Plate II, fig. 4), 5.2, 6.0, 6.2, 6.4, 6.9, 15.6), emphasized that recent mining operations in the Pseudocosmoceras sp. cf. masarowici Murashkin (27.0, quarry provided new information on lithology and 29.45), Pseudocosmoceras? sp. (34.5) attributed to the structure of the section, which influence its strati Pseudocosmoceras michalskii Zone. Preservation of graphic interpretation. For example, it appeared that bivalves is slightly better; their assemblage includes the the so called “belemnite level” of Bed 4 pinches out following taxa (identifications by V.A. Zakharov): along the strike. Correspondingly, this interval of the Meleagrinella echinata (Smith) (16.40, 19.30, 24.30, section should be interpreted as a lens being of local 29.35, 49.90, 53.00), Liostrea (Liostrea) mutiformis distribution and characterizing a local brief erosion (Koch) (42.10), Liostrea subgen. et sp. indet. (50.40), event. It should be noted that in our earlier works Thracia sp. indet. (50.40). Such a composition of fossil (Mitta and Sel’tser, 2002; Mitta et al., 2004) we noted assemblages indicates only Middle Jurassic age of host in Bed 5 containing Arcticoceras ishmae a bed with strata. The detailed description of the borehole along rounded belemnite rostra and other fossils, which was with accompanying geophysical and micropaleonto named in field descriptions as an “upper belemnite logical data will be published later. level.” Its distinct lateral variability and discontinuity prevented from defining it as an autonomous layer. Similarly, according to observations by Sel’tser, the DISCUSSION basal layer of siltstones, which was previously observ able above Bed 5, pinches out in the southeastern part As has been mentioned, the recently obtained data of the quarry. Such features are of importance prima on the Sokur quarry section neither confirm nor rily for the answer to the critique by (Meledina et al., refutes our previous inferences. Based on the analysis 2009). In this connection, it should also be noted that of published data and their investigations of Bajocian– new finds of fossils in beds 1–5 confirm or are consis Bathonian sections in northern East Siberia, tent with previous data. For example, Sel’tser found a Meledina et al. (2009) arrived at the conclusion that specimen of Parkinsonia sp. (sp. nov. ?) 250 mm in our views on the zonation and correlation of Bajo diameter approximately at the level of the cian–Bathonian sediments in the Saratov outskirts are Medvediceras masarowici find (probably, slightly erroneous. They also question the correctness of below). Inner whorls of this ammonite are covered by stratigraphic levels of our ammonite occurrences, pri relatively thin and densely spaced ribs typical of Par marily high boreal Arctocephalitinae forms, i.e., rep kinsonia forms from the West European Parkinsonia resentatives of the Arctocephalites and Arcticoceras parkinsoni Zone. Until this find, all the Parkinsonia genera. In their opinion, these ammonites should be forms from this interval of the section were juvenile located in the section above the “belemnite level” or specimens. Ivanov found among his fossils sampled in correspond to the latter. Moreover, beds 4 and 5 up to the 1990s from the “belemnite level an ammonite 1.5 m thick in total should correspond to the interval specimen with the matrix and taphonomic features of the Arctocephalites arcticus, Arctocephalites aff. indicating its origin precisely from this interval of the greenlandicus, Arcticoceras harlandi, and Arcticoceras section. This specimen (Plate II, fig. 1) is identified as ishmae zones in northern East Siberia. These Arcticoceras ishmae (v. Keyserlyng), an initial variety researchers assume that beds 4 and 5 are strongly con described by Callomon (1993) as morpha α. densed, and that the “belemnite level” reflects rework M.A. Grigor’ev donated us specimens of A. harlandi ing of the older Arctocephalites arcticus–A. greenland (Plate I, fig. 5) and A. ishmae found under and above icus zones” (Meledina et al., 2009, p. 68). No index the belemnite level,” respectively, as well as a well pre species of the last two zones have, however, been found served specimen of Arctocephalites freboldi Spath up to so far in the Saratov outskirts, and, of index species 160 mm in diameter. Unfortunately, the latter was col from other faunal horizons, only A. freboldi Spath, lected loose, although it eliminated doubts about the which characterizes the terminal faunal horizon of the validity of the identification of this species, which had Greenlandicus Zone, is found. However, their main previously been based on incomplete specimens. The arguments are based on the different molluscan last researcher also donated several Oraniceras speci assemblages characteristic of Siberia. mens from the Zone. Figure 2 only shows Besnosovi Our identifications of ammonites are not contested taxa found in situ. except for the imprint of an ammonite (Mitta and In 2003, A.V. Ivanov’s initiative was crowned by the Sel’tser, 2002, Plate 5, fig. 2) from the Ishmae Zone drilling of an exploration borehole in the Sokur quarry, determined as Parkinsonia s.l. Meledina et al., 2009 which recovered a sequence of gray clays approxi state that, in their opinion, this impression may mately 55 m thick below Bed 1. Unfortunately, core equally belong to a member of Arctocephalitinae. Par material sampled by V.B. Sel’tser yielded only frag kinsoniids became extinct in the middle part of the ments or juvenile shells of ammonites (identifications early Bathonian and their distribution above the Bes by V.V. Mitta; hereinafter, sampling depth (m) is given nosovi Zone is inconsistent with our views on correla in brackets): Parkinsoniidae gen. et. sp. indet. (2.5, tion of this zone with the Convergens and Macrescens 4.5, 27.8, 32.0, and 34.6), Parkinsonia sp. (4.0, 4.7 subzones of the Zigzag Zone of the standard scale.
STRATIGRAPHY AND GEOLOGICAL CORRELATION Vol. 19 No. 5 2011 THE UPPER BAJOCIAN–LOWER BATHONIAN BOUNDARY SECTION 507
Low. m Cal. ? 7 6
5 11 4 6 3
2 9 1 Middle–Upper Bathonian
5 ishmae β Arcticoc. ishmae β α ishmae α 4 Arcticoc. ishmae Arcticoc. harlandi, Arctoc. freboldi Oraniceras sp. Arcticoc. harlandi harlandi Oraniceras besnosovi Sokurella galaczi
Parkinsonia (s. l.) juv. 3 5 besnosovi Parkinsonia (s. l.) juv.
Oraniceras besnosovi, Oran. sp. juv. Oraniceras cf. besnosovi 2 mojarowskii 3 Oraniceras cf. mojarowskii Parkinsonia spp. juv.
Pseudocosm. masarowici, 1 Parkinsonia sp. (sp. nov.?) masarowici 1 Michalskii Besnosovi Ishmae Upper Bajocian Lower Bathonian
Fig. 2. The Bajocian–Bathonian Sokur quarry section, Saratov. (1) clay; (2) silt; (3) siltstone; (4) sideritic concretions; (5) “belemnite level;” (6) in situ ammonite finds. Scale bar 1 m. Abbrevi ations: (low. Cal.) Lower Callovian, (Arcticoc.) Arcticoceras, (Arctoc.) Arctocephalites, (Oran) Oraniceras, (Pseudocosm.) Pseudocosmoceras. Faunal horizons are shown on the right.
Therefore, we agree that the imprint under consider luscan groups could be heterochronous. This is partic ation represents a member of Arctocephalitinae. ularly obvious in case of brief or rapid (geologically Other notes in relation to the Saratov ammonites instanteous) events. In addition, available recent concern the position of arctocephalitines in the sec information on the geographic and stratigraphic dis tion (Figure 2 presents all the in situ finds) and the tribution of particular taxa is far from being complete consistency (or more accurately, the inconsistency) and final. The certain share of subjectivism in identifi between assemblages of ammonites, belemnites, and cation of fossils cannot be ruled out as well. bivalves (retrocerams) from the Saratov Volga region The main source of doubts for our opponents is the and from Middle Jurassic sections of North Siberia. It fact that almost all the Arctocephalites specimens are should be kept in mind that invasions of different mol found in situ, except for a single incomplete specimen
STRATIGRAPHY AND GEOLOGICAL CORRELATION Vol. 19 No. 5 2011 508 MITTA et al.
Plate I
3c
4b
1
3а 3b 2 5c
4а
5b 5а
STRATIGRAPHY AND GEOLOGICAL CORRELATION Vol. 19 No. 5 2011 THE UPPER BAJOCIAN–LOWER BATHONIAN BOUNDARY SECTION 509
Plate I. Cephalopods from the Lower Bathonian Sokur Road section, Saratov. All images are given 0.9 of natural size. (1–3) Cylindroteuthis spathi Sachs et Nalnjaeva: (1) Specimens SGM, no. CR 2884, (2) Specimen SGM, no. CR 2885, (3) Spec imen SGM, no. CR 2886, all the specimens originate from beds 3–5 without more exact position; (4) Pachyteuthis optima Sachs et Nalnjaeva, Specimen SGM, no. CR 2887, from beds 3–5 without more exact position; (5) Arcticoceras harlandi Rawson, Specimen PIN, phragmocone no. 5029/088, 0.5 m below the “belemnite level,” upper part of Bed 3 (Arcticoceras ishmae Zone, Arcticoceras harlandi faunal horizon). Collection by M.A. Grigor’ev. from the Ishmae Zone. Taking into consideration that known stratigraphic range of the genus Mesoteuthis Arctocephalites forms are ancestral relative to Arctico corresponds to the Pliensbachian–Aalenian in Europe ceras, their co occurrence may serve as a basis for sus and Pliensbachian–Bajocian in Siberia. The attribu picion of their reworked state or condensed section. At tion of Saratov species to this genus would make its the same time, the last Arctocephalites are also stratigraphic range unreasonably wide and the age of recorded in the Ishmae Zone of Greenland. When the host sediments, which is established as being examining the collection by J. Callomon and T. Birke undoubtedly Bathonian based on ammonites, older. lund in the Geological Museum of the University of In any case, new species are of little use for biostrati Copenhagen, V.V. Mitta was able to confirmed that. graphic correlation until they have been found in other Moreover, Mitta suggested that Cadoceratinae evolved areas. The same is also true of the placement of Sara from Arctocephalites in the middle Bathonian, while tov forms in the genus Brachybelus. It seems that it is Arcticoceras represents a dead end branch, which, like in fact more correct to treat these species as Nannobe many other early cardioceratids terminated its evolu lus (?) bellus and N. (?) parabellus until their internal tion with an suboxyconic shell. Nevertheless, Arcto structure can be studied. cephalites representatives are relatively rare in the Ish Our opponents (Meledina et al., 2009) consider mae Zone, and we assume that most specimens in our identifications of Paramegateuthis forms to be correct. collection originate from the upper part of the Besnos The occurrence of Paramegateuthis species correlates ovi Zone. The first elements of the high boreal fauna host layers with the base of the middle Bathonian, (retrocerams, see below) appear in this zone, and it is which is consistent with our data. logical to assume that high boreal cephalopods are In their opinion, rostra of Saratov Pachyteuthis sub also present at this level. rediviva belong to P. optima Sachs et Nalnjaeva and In the critique by Meledina et al. (2009), much ?Cylindroteuthis sp. It should be noted that species attention is paid to belemnites. I.S. Barskov described diagnostics of the group of species Pachyteuthis and illustrated (in Mitta et al., 2004) the species Nan optima, P. subrediviva, and P. tschernyschevi are diffi nobelus bellus Barskov, N. parabellus Barskov (the cult and extremely subjective (see discussion in Saks generic placement of these two species was only con and Nal’nyaeva, 1966, pp. 21, 27); most of their quan ditional), Paramegateuthis cf. pressa Nalnjaeva, P. cf. titative parameters are overlapping. Specimens illus manifesta Nalnjaeva, and Pachyteuthis subrediviva trated in Mitta et al. (2004) are attributed to P. subre (Lemoine). All the rostra illustrated in the above men diviva based on the more central position of the alveola tioned work originate from the Ishmae Zone (beds 4 and the rounded cross section of the rostrum in con and 5); in the opinion of Meledina et al. (2009), their trast to the rounded–trapezoid section characteristic identifications are doubtful. of P. optima. Additional material from this section Meledina et al. (2009) also do not agree that the (beds 1–5, mostly without more exact stratigraphic new species belong to the genus Nannobelus, consider position) donated by M.A. Grigor’ev includes rostra, ing them to be more closely related to Brachybelus or which may be attributed also to P. optima (Pate I, Mesoteuthis. Moreover, they note that Brachybelus fig. 4). In addition, this collection also contains Cylin forms are characteristic of the interval from the Toar droteuthis spathi Sachs et Nalnjaeva (Plate I, figs. 1–3). cian to the Lower Aalenian in Siberia and of the Bajo According to our opponents, the co occurrence of cian–Bathonian in Europe. Such different non over Paramegateuthis cf. manifesta Naln., P. cf. pressa lapping intervals suggest that the initial data might be Naln., Pachyteuthis optima Sachs et Naln., and ?Cylin faulty. It is conceivable that two new species attributed droteuthis species in Siberian sections is “characteris (conditionally and during the first description) to the tic of the Cylindroteuthis spathi belemnite zone that genus Nannobelus might belong to the other genus. comprises the interval from the Cranocephalites graci They are most similar to the species Mesoteuthis pyra lis to Arctocephalites aff. greenlandicus zones midalis (Zieten), which is reported from the upper (Meledina et al., 2009, p. 65). If this belemnite zone most Pliensbachian–basal Bajocian. At the same (Spathi) corresponds to three ammonite zones of the time, attribution of Saratov forms to the genus Meso Boreal scale (from the middle of the Bajocian to the teuthis seems groundless at least for two reasons: middle part of the Lower Bathonian), why can it not (1) most species of the genus, except for the above also include the next ammonite zone (Ishmae), partic mentioned one, are characterized by long subcylindri ularly taking into consideration that the Greenlandicus cal rostra (Saks and Nal’nyaeva, 1975, p. 42); (2) the Zone is defined with the moderator “aff,” i.e., with
STRATIGRAPHY AND GEOLOGICAL CORRELATION Vol. 19 No. 5 2011 510 MITTA et al.
Plate II
2 4
1b
3
1а
5 *
STRATIGRAPHY AND GEOLOGICAL CORRELATION Vol. 19 No. 5 2011 THE UPPER BAJOCIAN–LOWER BATHONIAN BOUNDARY SECTION 511
Plate II. Ammonites from the Upper Bajocian and Lower Bathonian Sokur section, Saratov. All the images are given 0.75 of natural size; asterisk designates the beginning of the living chamber. (1) (Arcticoceras ishmae (v. Keyserling) morpha alpha, Specimen PIN, no. 5029/089, phragmocone with the half whorl of the living chamber, Bed 4, “belemnite level” (Arcticoceras ishmae Zone, ishmae α)faunal horizon), collection by A.V. Ivanov; (2) Par kinsonia sp., Specimen PIN, no. 5029/092, deformed shell, Bed 3 1.5 m above the base (Lower Bathonian, Oraniceras besnosovi Zone, besnosovi faunal horizon); (3) Parkinsonia sp., Specimen PIN, no. 5029/093, deformed shell, top of Bed 1 (Upper Bajo cian, Pseudocosmoceras michalskii Zone); (4) Parkinsonia sp., Specimen PIN, no. 5029/090, deformed shell, Borehole Sokur skaya, depth of 4.7 m (Upper Bajocian, Pseudocosmoceras michalskii Zone, masarowici faunal horizon); (5) Oraniceras sp. juv., Specimen PIN, no. 5029/091, base of Bed 3 (Oraniceras besnosovi Zone, besnosovi faunal horizon). some conditionality? In addition, the holotype of a kina, 1963, p. 145), Retroceramus retrorsus (Keyser single species, which is identified by our opponents ling) in the Lena River basin is found in siltstones with from this assemblage in the binominal nomenclature Cranocephalites and sandstones with Arcticoceras; (Pachyteuthis optima), originates from the “Batho information on co occurrence of Arcticoceras ishmae nian–Callovian” interval (Saks and Nal’nyaeva, 1964, and Retroceramus retrorsus is also available from other p. 209) of the Izhma River section, most likely from publications from the second half of the 20th century. sandstones of the Ishmae Zone. Other rostra re iden It should be emphasized that species of Retrocera tified by V.I. Nal’nyaeva in open nomenclature can mus occur both at the “belemnite level” and are scat hardly be considered suitable for the dating of the host tered through the entire Ishmae Zone. The young sediments. specimen of R. ex gr. retrorsus illustrated in (Mitta et In fact, the comments of our opponents and their al., 2004, Plate 8, fig. 2) originates from the interval re identifications of belemnites, even if they are more located 1.0–1.5 m above the base of Bed 3 (Besnosovi correct, by no means disprove the age of sediments Zone). Sediments occurring 0.5–1.0 m above this base proposed in our stratigraphic model, despite discrep yielded Retroceramus sp. juv. (Mitta et al., 2004, p. 5). ancies in species and generic identifications. Small M.A. Grigor’ev donated to us an additional R. retror discrepancies in age estimates are most likely explain sus specimen from the Besnosovi Zone without indica able by insufficient information on the vertical distri tion of its exact stratigraphic position. Thus, the inva bution of belemnite taxa. sion of the Arctic fauna to the Saratov latitude took Of bivalves, our opponents commented on the place as early as the Early Bathonian Besnosovi phase, genus Retroceramus described from the Lower Batho and our assumptions on the distribution of Arctoceph nian interval: R. retrorsus (v. Keyserling), R. ex gr. ret alites representatives in this zone are quite appropriate rorsus (v. Keyserling), R. aff. polaris Koschelkina. taking into consideration the confinement of this These identifications were not contested. At the same genus mostly to older layers as compared with Arctico time, they note that “the Siberian R. retrorsus and ceras forms. overlying R. polaris zones comprise the interval from Taking into consideration the potential asynchrony the appearance of the ammonite genus Cranocepha of corresponding ammonite and belemnite assem lites to the Arctocephalites arcticus Zone” (Meledina blages in Bathonian sections of the Saratov Volga et al., 2009, p. 66), i.e. interval below beds with Arcti region and northern East Siberia, it follows from the coceras. Unfortunately, our opponents provide no ref comments of our opponents that it is of interest to erences to corresponding publications. According to compare the taxonomic composition of bivalve assem the monograph dedicated to retrocerams (Koschel blages from the Ishmae Zone in the Saratov outskirts
Table 1. The comparative analysis of the taxonomic composition of bivalve assemblages from the Lower Bathonian Arcti coceras ishmae Zone of the Timan–Pechora and East European paleobiogeographic provinces Saratov Volga region Izhma River basin
Liostrea (Deltostrea) eduliformis (Schlotheim) Mclearnia broenlundi (Ravn) Liostrea (Liostrea) multiformis (Koch) Liostrea (Liostrea) multiformis (Koch) Modiolus (Modiolus) aff. bipartitus (J. Sowerby) Modiolus (Striatomodiolus) sp. nov. Quenstedtia sp. ? Isocyprina sp. Protocardia sp. Mactromya aff. laevigata (Lahusen) Goniomya cf. v scripta (J. Sowerby) Pachymya (Arcomya) aff. sinuata (Agassiz) Pleuromya? sp. Gresslya lunulata Agassiz Pleuromya subpolaris Koschelkina
STRATIGRAPHY AND GEOLOGICAL CORRELATION Vol. 19 No. 5 2011 512 MITTA et al. and Izhma River basin (northern Pechora region). outskirts together with Arcticoceras. Nevertheless, Bivalves in the Izhma River basin (Dreshchanka no such species are present either in our or Creek) were sampled by V.V. Mitta (description of sec in Nal’nyaeva’s faunal lists; the last researcher had tions in Mitta, 2006, 2009; identifications by additional materials from the Sokur quarry at her dis V.A. Zakharov) (Table 1). posal. This supports heterochronous invasions of dif The comparative analysis of bivalve molluscan ferent molluscan groups to the Central Russian basin. assemblages from the Timan–Pechora and Central In the correlation model of Bajocian–Bathonian Russia early Bathonian seas of the Ishmae phase boundary units of Siberia (Meledina et al., 2009, reveals substantial differences. Despite the fact that fig. 2), the “belemnite level” of the Saratov outskirts is both coeval (based on ammonites) assemblages are correlated with the Manifesta belemnite subzone, the similar in their generic diversity (seven genera in the base of which corresponds to the base of the Arcticus Saratov Volga region vs. eight genera in the Pechora ammonite zone and lower boundary of the Bathonian River basin), they have only two genera and one spe Stage. At the same time, the authors note that “the cies in common: Liostrea (Liostrea) multiformis Medvediceras michalskii and Oraniceras besnosovi (Koch). It should, however, be noted that the entire zones attributed to the Upper Bajocian and lower half Lower Bathonian bivalve molluscan assemblage from of the Lower Bathonian, respectively, seem to be well the Saratov Volga region includes 26 species belonging grounded” (Meledina et al., 2009, p. 67). At the same to 23 genera (Mitta et al., 2004). Nevertheless, no gen time, if we admit the correctness of the correlation era and species occurring in the Saratov Volga region model proposed by our opponents, there is no place are found in the Pechora River basin or vice versa. The for the Besnosovi Zone (with typical early Bathonian following biogeographic situation seems to exist: the Oraniceras) within the Bathonian Stage. assemblage from the Timan–Pechora province We would like to receive answers for the following includes both Arctic (Mclearnia broenlundi (Ravn), questions: What are the grounds for the Bajoa Modiolus (Striatomodiolus) sp. nov., Pleuromya subpo cian/Bathonian boundary in the Siberian scale? What 2 are the arguments in favor of the boundary between the laris Koschelkina, Boreiothyris Dagys and low Boreal Cranocephalites carlsbergensis and Arctocephalites taxa (Liostrea (Liostrea) multiformis (Koch), Pachy arcticus in order to consider it to correspond to the mya (Acromyia) aff. sinuata (Agassiz), Gresslya lunu boundary between the Parkinsonia parkinsoni and Zig lata Agassiz), while the assemblage from the East zagiceras zigzag zones or the Bajocian–Bathonian European province is dominated only by typical low boundary in the West European scale? We were unable boreal taxa: Liostrea (Deltostrea) eduliformis (Schlo to find any answers to these questions in the publica theim), Liostrea (Liostrea) multiformis (Koch), Modi tions of our opponents. olus (Modiolus) aff. bipartitus (J. Sowerby), Goniomya cf. v scripta (J. Sowerby). Such a situation may most plausibly be explained by limited exchange between CONCLUSIONS benthic molluscan communities. Differences in the biogeographic structure of mollusks are also known The sequence of Middle Jurassic clays with sider from the Berriasian–Ryazanian Stage of the East itic carbonate concretions in the upper part is studied European Platform, where the Riasanites rjasanensis in the Saratov outskirts. The largest lower part of this Zone contains, along with local Craspeditidae, diverse sequence corresponds to the Pseudocosmoceras ammonites from the families Neocomitidae and michalskii Zone (an equivalent of Parkinsonia parkin Himalayitidae (immigrants from the Tethyan Ocean), soni Zone, a terminal Bajocian unit of the standard while most bivalve molluscan taxa are represented by scale) characterized exclusively by Peri Tethyan mol aboriginal forms (Zakharov and Mitta, 2010). lusks. It is overlain by the Oraniceras besnosovi Zone corresponding (by the Oraniceras range) to the West The critique by our Siberian colleagues (Meledina European Gonolkites convergens and Gonolkites macre et al., 2009), who emphasize that they are the authors scens subzones of the Zigzagiceras zigzag Zone. Its sed of parallel Middle Jurassic zonal scales based on vari iments also contain Arctic taxa (retrocerams and, pre ous molluscan groups, poses numerous natural ques sumably, the earliest Arctocephalites), in addition to tions, for which their article provides no answers. the Tethyan fauna. The Oraniceras besnosovi Zone is, For example, if the Harlandi/Ishmae interval cor in turn, overlain by the Arcticoceras ishmae Zone with responds to the Cylindroteuthis confessa and Pachy the mixed Boreal–Arctic bivalve assemblage and teuthis tschernyschevi belemnite zones, their index exclusively Arctic cephalopods. The occurrence of species (belemnites) should also occur in the Saratov intercalations with sideritic concretions and lenses with the rounded faunal remains indicate the strati 2 Boreiothyris Dagys, 1968 is a brachiopod genus (family Borei graphically condensed section and local erosion, thyridae Dagys, 1968) endemic for the Arctic paleobiogeo respectively, although no distinct marks of large sedi graphic region; it is reported from the stratigraphic range span ning from the Callovian to Kimmeridgian stages in Arctic Sibe mentary hiatuses are found in the section. In our opin ria (Dagys, 1968); according to our interpretation, its ion, all three above mentioned zones form a natural appearance should be dated back to the Bathonian Age. succession without any lacunae. Similar to the Oxycer
STRATIGRAPHY AND GEOLOGICAL CORRELATION Vol. 19 No. 5 2011 THE UPPER BAJOCIAN–LOWER BATHONIAN BOUNDARY SECTION 513 ites yeovilensis and Arisphinctes tenuiplicatus subzones European Russia, Izv. Vyssh. Uchebn. Zaved., Geol. Razved., of the Zigzag Zone in the West European zonal stan 2002, no. 6, pp. 14–18. dard, parkinsoniids are completely absent from the Atlas mezozoiskoi fauny i sporo pyl’tsevykh kompleksov Nizh Ishmae Zone. Taking into consideration the sedimen nego Povolzh’ya i sopredel’nykh oblastei. Vyp. 2. Golovonogie tary succession (the Besnosovi and Ishmae zones con mollyuski (Atlas of the Mesozoic Fauna and Palynological stitute an element of a single sedimentation cycle), we Complexes from the Lower Volga and Adjacent Regions. believe that the Ishmae Zone may be correlated with Issue 2. Cephalopods), Saratov: Izd vo Saratovsk. un ta, the upper part of the Lower Bathonian in the standard 1969, 275 p. scale (see correlation model in Mitta, 2009). Borisyak, A., Fauna of Donets Jurassic Sequences. 1. Ceph alopoda, Tr. Geol. Kom. Nov, Ser. 1908., no. 37, pp. 1–94. The critique by S.V. Meledina, T.I. Nal’nyaeva, Borisyak, A., Donets Jurassic Sequences, Geologiya Rossii, and B.N. Shurygin is largely based on revised identifi 1917, vol. 3, part 2, issue 3, 18 p. cations of belemnites and the uncertain position of Callomon, J.H., The Ammonite Succession in the Middle most Arctocephalites representatives found in the Jurassic of East Greenland, Bull. Geol. Soc. Denmark, 1993, Sokur quarry. Reidentification of some belemnites by vol. 40, pp. 83–113. no means contradicts inferences in Mitta et al. (2004): Dagys, A.S., Yurskie i rannemelovye brakhiopody severa despite discrepancies in species and generic identifica Sibiri (Jurassic and Early Cretaceous Brachiopods of North tions, the early Bathonian age of the host sediments is Siberia), Moscow: Nauka, 1968. confirmed. Arctocephalites forms may originate from Kamysheva Elpat’evskaya, V.G., Liftime Damages of both the lower part of the Ishmae Zone and from the Jurassic Ammonite Shells, Uchen. Zap. Saratovsk. Un ta. Besnosovi Zone. The last assumption is confirmed by Vyp. Geol, 1951, vol. 28, pp. 212–225. the distribution of Arctic Retroceramus bivalves in this Kamysheva Elpat’evskaya, V.G. and Ivanova, A.N., Atlas zone and finds of Arcticoceras harlandi in the upper rukovodyashchikh form iskopaemykh faun Saratovskogo Pov part of Bed 3. Finds of Arctocephalites freboldi indi olzh’ya (Atlas of Fossil Guide Forms from the Saratov Volga cates the presence of the terminal faunal horizon of Region), Saratov: Izd vo Saratovsk. un ta, 1947. the Greenlandicus Zone in the section. No guide spe Kamysheva Elpat’evskaya, V.G., Nikolaeva, V.P., and cies of other faunal horizons of this or the underlying Troitskaya, E.A., Opredelitel’ yurskikh ammonitov Sara Arctocephalites arcticus Zone have been found in the tovskogo Povolzh’ya (Guidebook of Ammonites from the outskirts of Saratov; consequently, the Arcticus and Saratov Volga Region), Moscow: Gosgeoltekhizdat, 1956. Greenlandicus ammonite zones are so far unknown in Kamysheva Elpat’evskaya, V.G., Nikolaeva, V.P., and the Volga region. Troitskaya, E.A., Jurassic Stratigraphy of the Saratov Volga River Right Side Based on Ammonites, in Stratigrafiya i fauna yurskikh i melovykh otlozhenii Saratovskogo Pov ACKNOWLEDGMENTS olzh’ya (Jurassic and Cretaceous Stratigraphy and Fauna of the Saratov Volga Region), Leningrad: GONTI, 1959, Some of the fossils discussed in this work were pp. 3–264. kindly donated by M.A. Grigor’ev (Saratov). Koshelkina, Z.V., Stratigrafiya i dvustvorchatye mollyuski The specimens illustrated were photographed by yurskikh otlozhenii Vilyuiskoi sineklizy i Priverkhoyanskogo V.T.Antonova and A.V. Mazin (Paleontological Insti kraevogo progiba (Jurassic Stratigraphy and Bivalves of the tute, Russian Academy of Sciences). We thank Vilyui Syneclise and Verkhoyansk Foredeep), Magadan: B.N. Shurygin for his valuable comments, which SVKNII, 1963. helped to improve the manuscript. This work was sup Mazarovich, A.N., Middle Jurassic Deposits of the Ilovlya ported by the Russian Foundation for Basic Research River Basin, Vestn. Mosk. Gorn. Akad, 1923, vol. 2, no. 1, (project no. 11 05 01122) and Program 25 of the Pre pp. 29–60. sidium of the Russian Academy of Sciences “Origin of Meledina, S.V., Nal’nyaeva, V.I., and Shurygin, B.N., Cor the biosphere and evolution of geobiological systems.” relation of Upper Bajocian–Bathonian Zones in Siberia We are grateful to all people and organizations who with the Stage Standard, Stratigr. Geol. Korrelyatsiya, 2009, vol. 17, no. 3, pp. 63–69 [Stratigr. Geol. Correlation (Engl. helped with this work. Transl.), vol. 17, no. 3, pp. 291–297]. Mitta, V.V., Sokurella galaczi gen. et sp. nov. and Other Reviewers T.B. Leonova, E.M. Tesakova, Middle Jurassic Parkinsoniidae (Ammonoidea) from the and B.N. Shurygin Lower Reaches of the Volga Region, Paleontol. Zh., 2004, no. 3, pp. 30–35 [Paleontol. J. (Engl. Transl.), no. 3, pp. 257–264]. 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